Why can't they live forever? RIP Hanz

My little buddy crossed the rainbow bridge yesterday, at the ripe old age of 16 and a half. He was my dog "by marriage" and I only knew him the last six years. But in that time he became my best friend and constant companion. My wife travels for work, and I work from home, so Hanz and I spent all the days together. The house feels so empty. I miss him so much.